How horizontal can be curved?

I'm not quite sure what you're asking here, but when you look at a horizon on Earth, the curvature of the Earth means that it is curved - however, that's confused a bit by other factors:

The lens in your eye has a curved surface so, like camera lenses, there's a degree of distortion inherent in the design. However, we don't see with our eyes, we see with our brain - and the brain "knows" the horizon should be straight and compensates.

If you wear spectacles then the lenses in them will also distort what you see - and the stronger the glasses, the more the distortion. I wear glasses and the horizon looks a little curved to me.